fix(i18n): correct usage label from "Total" to "Last 30d"

The usage stats query defaults to a 30-day rolling window, but the
UI label said "Total"/"累计" implying lifetime aggregation. Rename
to "Last 30d"/"近30天" so the label matches the actual query semantics.

Closes #1060

frontend/src/i18n/locales/en.ts

@@ -574,7 +574,7 @@ export default {
     groupRequired: 'Please select a group',
     usage: 'Usage',
     today: 'Today',
-    total: 'Total',
+    total: 'Last 30d',
     quota: 'Quota',
     lastUsedAt: 'Last Used',
     useKey: 'Use Key',
@@ -1308,7 +1308,7 @@ export default {
         actions: 'Actions'
       },
       today: 'Today',
-      total: 'Total',
+      total: 'Last 30d',
       noSubscription: 'No subscription',
       daysRemaining: '{days}d',
       expired: 'Expired',

frontend/src/i18n/locales/zh.ts

@@ -574,7 +574,7 @@ export default {
     groupRequired: '请选择分组',
     usage: '用量',
     today: '今日',
-    total: '累计',
+    total: '近30天',
     quota: '额度',
     lastUsedAt: '上次使用时间',
     useKey: '使用密钥',
@@ -1335,7 +1335,7 @@ export default {
         actions: '操作'
       },
       today: '今日',
-      total: '累计',
+      total: '近30天',
       noSubscription: '暂无订阅',
       daysRemaining: '{days}天',
       expired: '已过期',